Melodie Metzger

Biomechanical Evaluation of Pediatric Anterior Cruciate Ligament Reconstruction Techniques

Background: Anterior cruciate ligament (ACL) reconstruction rates in skeletally immature patients have risen recently because of increased injury frequency combined with growing awareness of the importance of treating them in an acute setting. Concerns over potential growth disturbances caused by traditional tunnel placement have prompted the description of several partial and complete physeal-sparing techniques. Hypothesis: Native knee kinematics will most closely be restored by the all-epiphyseal technique because it best re-creates the intra-articular ACL anatomy. Study Design: Controlled laboratory study. Methods: Six cadaveric knees were subjected to static anteroposterior, varus, and internal rotation forces at 0°,15°, 30°, 45°, 60°, and 90° of flexion. Displacement and rotation of the tibia with respect to the femur were measured in the intact knee, after ACL disruption, and again after ACL reconstruction using all-epiphyseal, transtibial over-the-top, and iliotibial band physeal-sparing techniques. Results: Peak anteroposterior translation in the ACL intact and deficient states was 2.8 ± 1.4 mm and 7.2 ± 2.7 mm, respectively, at 30°. The all-epiphyseal reconstruction had a peak translation of 5.1 ± 2.3 mm at 30°, and the transtibial over-the-top reconstruction had a peak of 4.8 ± 1.8 mm at 30°, both significantly greater than the ACL intact state. The iliotibial band technique had a peak anteroposterior translation of 1.7 ± 1.1 mm at 45°, which was significantly less than the ACL-deficient state. Internal rotation was significantly increased in the all-epiphyseal reconstruction compared with the ACL intact state and significantly decreased at all flexion angles except 0° in the iliotibial band reconstruction. The only technique to affect varus rotation was the iliotibial band reconstruction, which significantly decreased varus rotation from the ACL-deficient state at flexion angles greater than 30°. Conclusion: All physeal-sparing reconstruction techniques restored some stability to the knee. The iliotibial band reconstruction best restored anteroposterior stability and rotational control, although it appeared to overconstrain the knee to rotational forces at some flexion angles. Clinical Relevance: This study provides orthopaedic surgeons with objective knee kinematic data to help guide them in making more informed decisions on the optimal technique for ACL reconstruction in skeletally immature patients.

Minimizing errors associated with calculating the location of the helical axis for spinal motions

One of the more common comparative tools used to quantify the motion of the vertebral joint is the orientation and position of the (finite) helical axis of motion as well as the amount of translation along, and rotation about, this axis. A survey of recent studies that utilize the helical axis of motion to compare motion before and after total disc replacement reveals a lack of concern for the relative errors associated with this metric. Indeed, intrinsic algorithmic and experimental errors that arise when interpreting motion tracking data can easily lead to a misinterpretation of the changes caused by replacement disc devices. While previous studies examining these errors exist, most have overlooked the errors associated with the determination of the location of the helical axis and its intersection with a chosen plane. The purpose of the study presented in this paper was to evaluate the sensitivity and reliability of the helical axis of motion as a comparative tool for kinematically evaluating spinal prostheses devices. To this end, we simulated a typical spine biomechanics testing experiment to investigate the accuracy of calculating the helical axis and its associated parameters using several popular algorithms. The resultant data motivated the development of a new algorithm that is a hybrid of two existing algorithms. The improved accuracy of this hybrid method made it possible to quantify some of the changes to the kinematics of a spinal unit that are induced by distinct placements of a total disc replacement.

On the Stiffness Matrix of the Intervertebral Joint: Application to Total Disk Replacement

The traditional method of establishing the stiffness matrix associated with an intervertebral joint is valid only for infinitesimal rotations, whereas the rotations featured in spinal motion are often finite. In the present paper, a new formulation of this stiffness matrix is presented, which is valid for finite rotations. This formulation uses Euler angles to parametrize the rotation, an associated basis, which is known as the dual Euler basis, to describe the moments, and it enables a characterization of the nonconservative nature of the joint caused by energy loss in the poroviscoelastic disk and ligamentous support structure. As an application of the formulation, the stiffness matrix of a motion segment is experimentally determined for the case of an intact intervertebral disk and compared with the matrices associated with the same segment after the insertion of a total disk replacement system. In this manner, the matrix is used to quantify the changes in the intervertebral kinetics associated with total disk replacements. As a result, this paper presents the first such characterization of the kinetics of a total disk replacement.

Biomechanical Analysis of the Proximal Adjacent Segment after Multilevel Instrumentation of the Thoracic Spine: Do Hooks Ease the Transition?

Study Design Biomechanical cadaveric study. Objective Clinical studies indicate that using less-rigid fixation techniques in place of the standard all-pedicle screw construct when correcting for scoliosis may reduce the incidence of proximal junctional kyphosis and improve patient outcomes. The purpose of this study is to investigate whether there is a biomechanical advantage to using supralaminar hooks in place of pedicle screws at the upper-instrumented vertebrae in a multilevel thoracic construct. Methods T7–T12 spines were biomechanically tested: (1) intact; (2) following a two-level pedicles screw fusion from T9 to T11; and after proximal extension of the fusion to T8–T9 with (3) bilateral supra-laminar hooks, (4) a unilateral hook + unilateral screw hybrid, or (5) bilateral pedicle screws. Specimens were nondestructively loaded while three-dimensional kinematics and intradiscal pressure at the supra-adjacent level were recorded. Results Supra-adjacent hypermobility was reduced when bilateral hooks were used in place of pedicle screws at the upper-instrumented level, with statistically significant differences in lateral bending and torsion (p < 0.05 and p < 0.001, respectively). Disk pressures in the supra-adjacent segment were not statistically different among top-off techniques. Conclusions The use of supralaminar hooks at the top of a multilevel posterior fusion construct reduces the stress at the proximal uninstrumented motion segment. Although further data is needed to provide a definitive link to the clinical occurrence of PJK, this in vitro study demonstrates the potential benefit of “easing” the transition between the stiff instrumented spine and the flexible native spine and is the first to demonstrate these results with laminar hooks.

The relationship between serum vitamin D levels and spinal fusion success: a quantitative analysis.

Study Design. An in vivo dosing study of vitamin D in a rat posterolateral spinal fusion model with autogenous bone grafting. Rats randomized to 4 levels of vitamin D–adjusted rat chow, longitudinal serum validation, surgeons/observers blinded to dietary conditions, and rats followed prospectively for fusion endpoint. Objective. To assess the impact of dietary and serum levels of vitamin D on fusion success, consolidation of fusion mass, and biomechanical stiffness after posterolateral spinal fusion procedure. Summary of Background Data. Metabolic risk factors, including vitamin D insufficiency, are often overlooked by spine surgeons. Currently, there are no published data on the causal effect of insufficient or deficient vitamin D levels on the success of establishing solid bony union after a spinal fusion procedure. Methods. Fifty rats were randomized to 4 experimentally controlled rat chow diets: normal control, vitamin D–deficient, vitamin D–insufficient, and a nontoxic high dose of vitamin D, 4 weeks prior to surgery and maintained postsurgery until sacrifice. Serum levels of 25(OH)D were determined at surgery and sacrifice using radioimmunoassay. Posterolateral fusion surgery with tail autograft was performed. Rats were sacrificed 12 weeks postoperatively, and fusion was evaluated via manual palpation, high-resolution radiographs, micro–computed tomographic scans, and biomechanical testing. Results. Serum 25(OH)D and calcium levels were significantly correlated with vitamin D–adjusted chow (P < 0.001). There was a dose-dependent relationship between vitamin D–adjusted chow and manual palpation fusion, with greatest differences found in measures of radiographical density between high and deficient vitamin D (P < 0.05). Adequate levels of vitamin D (high and normal control) yielded stiffer fusion than inadequate levels (insufficient and deficient) (P < 0.05). Conclusion. Manual palpation fusion rates increased with supplementation of dietary vitamin D. Biomechanical stiffness, bone volume, and density were also positively related to vitamin D and calcium. Level of Evidence: N/A

The Effect of Hamstring Tendon Autograft Harvest on the Restoration of Knee Stability in the Setting of Concurrent Anterior Cruciate Ligament and Medial Collateral Ligament Injuries

Background: A hamstring autograft is commonly used in anterior cruciate ligament (ACL) reconstruction (ACLR); however, there is evidence to suggest that the tendons harvested may contribute to medial knee instability. Hypothesis: We tested the hypothesis that the gracilis (G) and semitendinosus (ST) tendons significantly contribute to sagittal, coronal, and/or rotational knee stability in the setting of ACLR with a concurrent partial medial collateral ligament (MCL) injury. Study Design: Controlled laboratory study. Methods: Twelve human cadaveric knees were subject to static forces applied to the tibia including an anterior-directed force as well as varus, valgus, and internal and external rotation moments to quantify laxity at 0°, 30°, 60°, and 90° of flexion. The following ligament conditions were tested on each specimen: (1) ACL intact/MCL intact, (2) ACL deficient/MCL intact, (3) ACL deficient/partial MCL injury, and (4) ACLR/partial MCL injury. To quantify the effect of muscle loads, the quadriceps, semimembranosus, biceps femoris, sartorius (SR), ST, and G muscles were subjected to static loads. The loads on the G, ST, and SR could be added or removed during various test conditions. For each ligament condition, the responses to loading and unloading the G/ST and SR were determined. Three-dimensional positional data of the tibia relative to the femur were recorded to determine tibiofemoral rotations and translations. Results: ACLR restored anterior stability regardless of whether static muscle loads were applied. There was no significant increase in valgus motion after ACL transection. However, when a partial MCL tear was added to the ACL injury, there was a 30% increase in valgus rotation (P < .05). ACLR restored valgus stability toward that of the intact state when the G/ST muscles were loaded. A load on the SR muscle without a load on the G/ST muscles restored 19% of valgus rotation; however, it was still significantly less stable than the intact state. Conclusion: After ACLR in knees with a concurrent partial MCL injury, the absence of loading on the G/ST did not significantly alter anterior stability. Simulated G/ST harvest did lead to increased valgus motion. These results may have important clinical implications and warrant further investigation to better outline the role of the medial hamstrings, particularly among patients with a concomitant ACL and MCL injury. Clinical Relevance: A concurrent ACL and MCL injury is a commonly encountered clinical problem. Knowledge regarding the implications of hamstring autograft harvest techniques on joint kinematics may help guide management decisions.

Biomechanical analysis of lateral interbody fusion strategies for adjacent segment degeneration in the lumbar spine

BACKGROUND CONTEXT Surgical treatment of symptomatic adjacent segment disease (ASD) typically involves extension of previous instrumentation to include the newly affected level(s). Disruption of the incision site can present challenges and increases the risk of complication. Lateral-based interbody fusion techniques may provide a viable surgical alternative that avoids these risks. This study is the first to analyze the biomechanical effect of adding a lateral-based construct to an existing fusion. PURPOSE The study aimed to determine whether a minimally invasive lateral interbody device, with and without supplemental instrumentation, can effectively stabilize the rostral segment adjacent to a two-level fusion when compared with a traditional posterior revision approach. STUDY DESIGN/SETTING This is a cadaveric biomechanical study of lateral-based interbody strategies as add-on techniques to an existing fusion for the treatment of ASD. METHODS Twelve lumbosacral specimens were non-destructively loaded in flexion, extension, lateral bending, and torsion. Sequentially, the tested conditions were intact, two-level transforaminal lumbar interbody fusion (TLIF) (L3-L5), followed by lateral lumbar interbody fusion procedures at L2-L3 including interbody alone, a supplemental lateral plate, a supplemental spinous process plate, and then either cortical screw or pedicle screw fixation. A three-level TLIF was the final instrumented condition. In all conditions, three-dimensional kinematics were tracked and range of motion (ROM) was calculated for comparisons. Institutional funds (<

Biomechanical Evaluation of Pediatric Anterior Cruciate Ligament (acl) Reconstruction Techniques With and Without the Anterolateral Ligament (all)

50,000) in support of this work were provided by Medtronic Spine. RESULTS The addition of a lateral interbody device superadjacent to a two-level fusion significantly reduced motion in flexion, extension, and lateral bending (p<.05). Supplementing with a lateral plate further reduced ROM during lateral bending and torsion, whereas a spinous process plate further reduced ROM during flexion and extension. The addition of posterior cortical screws provided the most stable lateral lumbar interbody fusion construct, demonstrating ROM comparable with a traditional three-level TLIF. CONCLUSIONS The data presented suggest that a lateral-based interbody fusion supplemented with additional minimally invasive instrumentation may provide comparable stability with a traditional posterior revision approach without removal of the existing two-level rod in an ASD revision scenario.

Biomechanical Analysis of a Novel Buried Fixation Technique Using Headless Compression Screws for the Treatment of Patella Fractures

BACKGROUND Two popular physeal-sparing procedures used in the management of anterior cruciate ligament (ACL) injuries in skeletally immature patients are the iliotibial band (ITB) ACL reconstruction (ACLR) and the all-epiphyseal (AE) ACLR. Although there has been concern for overconstraint of the lateral compartment of the knee with the ITB ACLR technique, rotational stability, as provided by the anterolateral ligament (ALL) and ACL, has not been assessed in the setting of pediatric ACLR techniques. Our hypothesis is that the ITB ACLR and AE ACLR with ALL reconstruction (ALLR) will best replicate the biomechanical profile of the intact ACL that is lost with transection of the ACL and ALL. METHODS Eight cadaveric legs were statically loaded with an anterior drawer force and varus, valgus, internal and external rotational moments at 0, 30, 60, and 90 degrees of flexion. Displacement and rotation were recorded in the following conditions: intact ACL/intact ALL, ACL-deficient/intact ALL, ITB ACLR/intact ALL, ITB ACLR/ALL-deficient, ACL-deficient/ALL-deficient, AE ACLR/ALL-deficient, AE ACLR/ALLR. RESULTS Both ACLR techniques reduced anterior tibial translation from the ACL-deficient state, but neither restored it to the intact state (P<0.05), except in full extension. ALL deficiency increased anterior tibial translation in the ACL-deficient state (P<0.05). In rotational testing, no significant increase was seen with transection of the ACL, but the ACL-deficient/ALL-deficient state had a significant increase in internal rotation (P<0.05). This was significantly restored to the intact state at most flexion angles with the ITB ACLR without rotational overconstraint of the lateral compartment. The AE ACLR/ALL-deficient state and AE ACLR/ALLR improved rotational stability at lower flexion angles, but not at 60 and 90 degrees. There were no significant changes in varus/valgus moments. CONCLUSIONS In this model, the ITB ACLR provided the superior biomechanical profile between our tested reconstructions. It best corrected both AP and rotatory stability without overconstraining the knee. The AE ACLR and AE ACLR/ALLR improved both parameters but not at all flexion angles and not as robustly. ACL deficiency in the knee increased anterior tibial translation, but did not affect rotatory stability. ALL deficiency in the knee increased anterior displacement and rotational moments in the ACL-deficient state. CLINICAL RELEVANCE Cadaveric Laboratory Study. The ITB ACLR seems to be the biomechanically superior pediatric ACLR technique to regain translational and rotational stability.BACKGROUND Two popular physeal-sparing procedures used in the management of anterior cruciate ligament (ACL) injuries in skeletally immature patients are the iliotibial band (ITB) ACL reconstruction (ACLR) and the all-epiphyseal (AE) ACLR. Although there has been concern for overconstraint of the lateral compartment of the knee with the ITB ACLR technique, rotational stability, as provided by the anterolateral ligament (ALL) and ACL, has not been assessed in the setting of pediatric ACLR techniques. Our hypothesis is that the ITB ACLR and AE ACLR with ALL reconstruction (ALLR) will best replicate the biomechanical profile of the intact ACL that is lost with transection of the ACL and ALL. METHODS Eight cadaveric legs were statically loaded with an anterior drawer force and varus, valgus, internal and external rotational moments at 0, 30, 60, and 90 degrees of flexion. Displacement and rotation were recorded in the following conditions: intact ACL/intact ALL, ACL-deficient/intact ALL, ITB ACLR/intact ALL, ITB ACLR/ALL-deficient, ACL-deficient/ALL-deficient, AE ACLR/ALL-deficient, AE ACLR/ALLR. RESULTS Both ACLR techniques reduced anterior tibial translation from the ACL-deficient state, but neither restored it to the intact state (P<0.05), except in full extension. ALL deficiency increased anterior tibial translation in the ACL-deficient state (P<0.05). In rotational testing, no significant increase was seen with transection of the ACL, but the ACL-deficient/ALL-deficient state had a significant increase in internal rotation (P<0.05). This was significantly restored to the intact state at most flexion angles with the ITB ACLR without rotational overconstraint of the lateral compartment. The AE ACLR/ALL-deficient state and AE ACLR/ALLR improved rotational stability at lower flexion angles, but not at 60 and 90 degrees. There were no significant changes in varus/valgus moments. CONCLUSIONS In this model, the ITB ACLR provided the superior biomechanical profile between our tested reconstructions. It best corrected both AP and rotatory stability without overconstraining the knee. The AE ACLR and AE ACLR/ALLR improved both parameters but not at all flexion angles and not as robustly. ACL deficiency in the knee increased anterior tibial translation, but did not affect rotatory stability. ALL deficiency in the knee increased anterior displacement and rotational moments in the ACL-deficient state. CLINICAL RELEVANCE Cadaveric Laboratory Study. The ITB ACLR seems to be the biomechanically superior pediatric ACLR technique to regain translational and rotational stability.

Effect of modulating dietary vitamin D on the general bone health of rats during posterolateral spinal fusion: VITAMIN D IN RAT FUSION

The traditional technique for patella fracture fixation utilizes prominent hardware. Prominent hardware use, however, results in a high rate of reoperation for symptomatic implant removal. This biomechanical study evaluates the effectiveness of a novel patella fixation technique that minimizes implant prominence. Patellar transverse osteotomies were created in 13 pairs of cadaveric knees. Paired knees were assigned to either standard fixation (SF) using cannulated partially threaded screws and stainless steel wire tension band, or buried fixation (BF) using headless compression screws with a No. 2 FiberWire tension band and a No. 5 FiberWire cerclage suture. Quadriceps tendons were cyclically loaded to full extension followed by load to failure. The gap across the fracture site, stiffness, and load to failure were measured. The differences in stiffness and load to failure between the 2 groups were not statistically significant. During cyclic loading, significantly greater gapping was observed across the fracture site in the BF group compared with SF group (P < .05). Both constructs failed under loads that exceeded typical loads experienced during the postoperative rehabilitation period. Nevertheless, the BF technique demonstrated larger gap formation and a reduced load to failure than the SF technique. Further clinical studies are therefore underway to determine whether the use of constructs with decreased stability but increased patient comfort could improve clinical outcomes and reduce reoperation rates.